1. Field of the Invention
The invention relates to the exchange of hydrogen isotopes between gaseous hydrogen and liquid water. More particularly, it relates to an enhanced catalytic process for enhancing said exchange and desired deuterium concentration.
2. Description of the Prior Art
Deuterium oxide, or heavy water, is used primarily as a moderator for various nuclear reactor designs. Because of their abundance, natural water sources comprise the major source for such heavy water. Since the natural concentration of deuterium oxide in water is generally only about 0.147 mole percent, extensive concentration of said deuterium is required for practical commercial operations. The initial concentration of deuterium, e.g. in the first and/or second stage of concentration, is found to expend the most energy for separation, and it is such initial concentration to which the invention pertains.
The use of a dual temperature isotope exchange system for heavy water concentration is known in the art as evidenced by the Spevack U.S. Pat. No. 2,895,803, relating to the so-called GS (Girdler-Sulfide) process. Two serially arranged gas-liquid contacting towers are employed in this process, one operating at about 130.degree. C. and 300 psia. A feedwater stream passes sequentially through the cold tower, a dehumidification section, the hot tower and a humidification section countercurrent to an upwardly flowing stream of hydrogen sulfide gas therein. The water is progressively enriched in deuterium as it flows downwardly through the hot tower. Conversely, the hydrogen sulfide stream is enriched in deuterium as it passes upwardly in the hot tower and is depleted therein as it passes upwardly through the cold tower. The concentration of deuterium in each stream is maximized between the hot and cold towers. In the conventional GS process, a portion of the enriched water and gas between the towers is withdrawn for further processing in order to further concentrate the heavy water product. Water is discharged to waste from the bottom of the hot tower, while the hydrogen sulfide is continuously recycled through essentially a closed loop circulation path.
Various improvements have been made to the basic GS process over the years, particularly as disclosed in the Spevack patent, U.S. Pat. No. 3,860,698. This modification utilizes the closed-cycle circulation not only of the gaseous hydrogen sulfide exchange fluid, but also of a portion of the liquid feed water contacted therewith. Thus, water is feed from a source of supply to a hot tower feed section for contact against an upwardly flowing hydrogen sulfide gas stream, with said feed water being restricted solely to said hot tower in which it becomes depleted in the desired isotope, deuterium, as it flows downward therein, while the upwardly flowing hydrogen sulfide gas becomes enriched in the deuterium isotope. The remaining portions of the contacting system are serviced by an essentially closed liquid water recirculation loop. The closed loop water, after cooling, enters the top of the cold tower and descends downwardly therein for contact against the upwardly flowing hydrogen sulfide gas stream that becomes enriched with the desired deuterium isotope. Upon leaving the cold tower, the water enriched in deuterium enters the top of a dehumidifying section that serves to remove water vapor from, and to cool, the upwardly flowing hydrogen sulfide gas stream entering said section from the hot tower. The liquid water is, in turn, heated therein so that its temperature is elevated sufficiently for it to be subsequently fed to the top section of the hot tower. If desired, a portion of this heated water is withdrawn and fed to the second concentration stage, while another portion is subjected to several stages of heat exchange to appropriately cool the liquid for its use as additional heat exchange fluid for the cooling of the hydrogen sulfide gas stream in the dehumidifying section.
In this modification of the GS process, the previously heated liquid water fed to the top section of the hot tower passes downwardly therein countercurrently against the upwardly flowing hydrogen sulfide gas stream in said portion of the hot tower. At its elevated temperature, the liquid water becomes depleted in the isotope deuterium, while the hydrogen sulfide becomes enriched in this isotope. The hot liquid water withdrawn from the bottom of the hot tower top section is divided into three streams. The first, major portion of the water is withdrawn, cooled in a heat exchange zone, and fed to the top of the cold tower, i.e. recycled in the closed liquid loop. The second portion of the hot liquid is withdrawn and fed to the top of a hot tower recycle section for further contacting with an upwardly flowing hydrogen sulfide gas stream. A third, minor portion of the hot liquid water is fed to the top of the hot tower feed section to ensure that any feed water entrained in the upwardly flowing gas stream within said hot water feed section is not carried thereby into the top section of the hot tower. The hot liquid water contacted against the upwardly flowing hydrogen sulfide gas in the hot tower recycle section is further depleted in the desired isotope and is withdrawn from the section and fed to a humidifying section. In the embodiment of FIG. 2 of said U.S. Pat. No. 3,860,698, the hot tower recycle section is not employed and, assumably, one portion of the liquid water from the hot tower top section would be fed directly to the humidifying section, which is used to prepare the cold hydrogen sulfide gas stream recirculated from the top of the cold tower with appropriate heating and humidification for subsequent feeding to the bottom of the hot tower feed section. A portion of the heated and humidified hydrogen sulfide gas stream is fed to the bottom of the hot tower recycle section, if employed. The cooled liquid water withdrawn from the bottom of the humidifying section is recycled through an appropriate heat exchange zone, for further cooling, to the top of the cold tower.
The Spevack U.S. Pat. No. 3,860,698, discloses the essentially complete separation of the feed water from the remaining liquid water used in the process. Mixing occurs only through the indicated use of a small portion of the hot liquid withdrawn from the bottom of the hot tower top section for the desired suppression of entrained liquid in the hot tower feed section. The patent indicates that the modified GS process disclosed therein has particular utility when the feed water constitutes a highly corrosive material such as sea water. In the practice of the modified process, expensive materials of construction need only be used because of feed water corrosion in the hot tower feed section. The patent also indicates that the process is particularly useful when there is a limited supply, or a high cost supply of feed substances.
Regardless of the usefulness of the basic GS process and of the improvements therein disclosed by Spevack, inherent drawbacks exist in the water-hydrogen sulfide process that can not be avoided. For instance, hydrogen sulfide is a very poisonous gas that requires extraordinary care and safety precautions in its handling. In addition, the hydrogen sulfide, in combination with materials present in the feed water, creates serious corrosion and foaming problems. The corrosion problem can only be overcome by employing more expensive materials of construction. The foaming problem is typically handled by the addition of various chemical antifoam agents to the water, although expensive pretreatment operations can also minimize the foaming problem. In addition, pollution problems are caused by the hydrogen sulfide, since it is dissolved in the wastewater along with any chemical agents employed to combat said foaming problem. As a result, expensive post-treatment operations are also necessary as part of the overall deuterium concentration operation.
The water-hydrogen sulfide isotopic exchange process is also subject to significant physical constraints. The upper pressure limit of this system is limited to about 300 psia because of the possibility of hydrogen sulfide condensation in the cold tower. Additionally, the hot tower temperature is also somewhat constrained because of the excessive humidification and dehumidification requirements accompanying high temperature operation at the limiting pressure. The cold tower operating temperature is limited by the formation of ice and hydrogen sulfide solid hydrate. Both tower limits thereby fix the temperature difference between the towers, and thus limit the theoretical degree of separation obtainable in the isotopic exchange process. Such physical bounds on temperature and pressure, taken together, limit the theoretical productivity of the GS process. When these limitations are coupled with the relatively low equilibrium constant of the hydrogen sulfide-water exchange reaction, it will be seen that drawbacks exist in the commercial concentration of deuterium for heavy water production by means of the GS process. This process has nevertheless been employed in commercial operations because the alternative routes to heavy water production have been even more expensive or less efficient than the GS process.
In an effort to overcome such inherent limitations or problems in the established commercial practice for deuterium concentration, considerable attention has been given to the hydrogen-water isotopic exchange for deuterium concentration and heavy water production, since the equilibrium constant for such exchange is significantly higher than for the hydrogen sulfide-water exchange from the GS process. In the hydrogen-water exchange system, the isotopic exchange proceeds according to a two-step reaction process in which the following reactions occur simultaneously: EQU HD+H.sub.2 O.sub.(g) .fwdarw.H.sub.2 +HDO.sub.(g), (1)
and EQU HDO.sub.(g) +H.sub.2 O.sub.(e) .fwdarw.H.sub.2 O.sub.(g) +HDO.sub.(1)( 2)
Under normal conditions, however, the rate of exchange reaction (1) of this two-step process is excessively low, and, as such, creates a major obstacle to the commercialization of the process. Efforts have been made, therefore, to develop suitable materials for catalyzing said reaction (1), as well as means for extending the lifetime of useful catalysts for effective service in commercial deuterium concentration operations. Catalyst materials heretofore found potentially useful for such operations are extremely sensitive, however, to fouling by flooding with water used in the hydrogen-water isotopic exchange system. As a result, extremely complicated process designs were initially proposed for the use of such catalysts in hydrogen-water systems. Such processes commonly would require that a portion of the water, as vapor, be repeatedly superheated, contacted with the hydrogen in the presence of the catalyst to enrich the deuterium concentration, and then contacted with liquid water to effect a transfer of the deuterium to the liquid phase. Such processing would be exceedingly energy intensive, and catalyst fouling would remain as a significant problem. More recently, attempts have been made to produce hydrophobic catalysts useful for the hydrogen-water exchange so that the catalyst system need not be physically isolated from the stream of liquid water, while catalyst fouling could nevertheless be retarded or effectively avoided.
One approach for the providing of a hydrophobic catalyst system for enhancing the isotopic exchange reaction between hydrogen and water utilizes a porous catalytic support structure, e.g. alumina or charcoal, upon which a catalytic material selected from the Group VIII metals is deposited, with a water-proof coating provided over the entire sub-structure by a silicone resin or a film of polytetrafluoroethylene. The coating ensures that the catalytic material is accessible to the hydrogen gas as required for the isotopic exchange of reaction (1) above, but is substantially inaccessible to liquid water, i.e. the catalyst is waterproof, to avoid fouling of the catalyst by the water in the hydrogen-water system. Such a hydrophobic catalytic system is described in the Stevens U.S. Pat. No. 3,888,974.
A catalyst system that can be so employed without the need for physical isolation thereof from the stream of liquid water is the Moxy System of The Mead Corporation. This System, which has important and wide application, serves to promote chemical reactions and/or mass transfer between entities comprising or contained in fluids, wherein either (1) solid material porous on at least a molecular scale or (2) one or more solid heterogenous, porous or non-porous, catalysts operate in a medium of at least two immiscible fluid phase, the surface of such solid material or catalyst being treated so that part is wetted by one fluid phase and part by another immiscible fluid phase, thereby ensuring that respective portions of such solid material or catalyst are directly in contact with one or the other of the immiscible fluid phase, and wherein such entities are in contact with each other and with such solid material or catalyst. Further information concerning the Mead Moxy System can be found in Smith et al, U.S. Pat. No. 4,054,419, and Canadian Patents Nos. 944,535 and 959,821, all of which relate generally to reduction-oxidation processes and apparatus utilizing in the preferred form a particulate carbon discontinuously coated with polytetrafluoroethylene.
Another means of assuring a waterproof or hydrophobic catalyst for the desired isotopic exchange was disclosed by Rolston et al, U.S. Pat. No. 4,025,560, in which the waterproofing or hydrophobic material, with polytetrafluoroethylene or carbon black being disclosed, serves as the support structure for the catalytic material that is selected from the Group VIII elements and is deposited directly on the hydrophobic support. Butler et al, U.S. Pat. No. 4,126,667, describes a packed tower system in which a catalytic material having a permeable coating is interspersed with a non-catalytic packing material provided with a hydrophilic coating. Butler et al, U.S. Pat. No. 4,143,123, describes still another catalytic system in which partially platinized carbon particles are dispersed in a polytetrafluoroethylene matrix in a weight ratio of 1:1 to 3:1 of polytetrafluoroethylene: partially platinized carbon particles. The inherently hydrophobic, porous tetrafluoroethylene matrix purportedly allows the platinum to catalyze the exchange reaction in the presence of liquid water while retarding catalytic fouling.
Catalytic systems are thus available that would serve to enhance the commercial feasibility of employing a hydrogen-water system for deuterium concentration purposes. Such systems have commonly been taught as being disposed as a packed column or bed. The packing may comprise discrete bodies of the catalytic material or, alternatively, the bodies of catalytic material may themselves be supported on a distinct packing material, such as well-known Rashig rings, Lessing rings, or Berl saddles, with either co-current or countercurrent flow of liquid and gas through the packed column. It has also been proposed to use water soluble materials to catalyze the hydrogen-water exchange reaction. Mills, U.S. Pat. No. 2,967,089, thus discloses a homogeneous system for the low temperature isotopic exchange. Hydrogen and water are contacted in the presence of a solution of a complex cobalt-cyanide salt of an alkali metal containing the cobalt in a mono-valent or di-valent state. It has further been proposed that a heterogeneous catalyst be employed in the form of a solid-liquid suspension in water under high pressure. E. W. Becker et al, "Enrichment of Heavy Water by High-Pressure Exchange Between Hydrogen and an Aqueous Suspension of a Catalyst", Proceedings of the Second United Nations International Conference on the Peaceful Use of Atomic Energy, Volume 4: Production of Nuclear Materials and Isotopes, United Nations Publications 1958, pp. 543-549, disclose such a suspension in a system employing an entirely closed liquid loop. This approach, in which hydrogen gas provides the isotope source and in which platinized activated charcoal was disclosed as the catalyst, requires a large amount of hydrogen to drive the process.
In the hydrogen-water exchange for deuterium concentration, it will be seen that the use of hydrophobic coatings on the exchange catalyst is well known in the art. While such coatings serve to lengthen the useful lifetime of the catalyst by preventing rapid fouling by liquid water, other problems are found to exist in such isotopic exchange processing that also tend to limit catalyst lifetime. For example, catalyst poisons, such as scale forming compounds, present in the feed water will tend to slowly retard catalyst activity. Expensive feed pretreatment is needed to minimize this problem. In addition, catalyst attrition with the waste water stream, resulting from catalyst erosion, must be countered by expensive post treatment practices. While the slurry approach described by Becker et al, as referred to above, provides a potential solution to these problems, the need for a large supply source of hydrogen therefor is a major drawback that cannot be avoided in the slurry approach.
It is an object of the invention, therefore, to provide an improved hydrogen-water isotopic exchange process for deuterium concentration.
It is another object of the invention to provide a dual temperature, hydrogen-water exchange process having productivity limits significantly in excess of those of the GS hydrogen sulfide-water exchange process.
It is another object of the invention to provide a dual temperature, isotopic exchange process in which a catalyst can be effectively employed to increase the rate of hydrogen-water exchange without undue fouling by said water.
It is another object of the invention to provide a process of isotopic exchange employing a hydrogen-water system with increased deuterium recovery from a feed stream relative to the GS process for hydrogen sulfide-water exchange without increase in the quantity of feed substance processed.
It is a further object of the invention to provide a dual temperature, hydrogen-water isotopic exchange process in which a valuable catalyst can be used for long periods of operation without excessive deactivation or attrition.
It is a further object of the invention to provide a dual temperature isotopic exchange process for deuterium concentration in which valuable catalyst material is protected from feed water impurities tending to retard catalyst activity.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features of which are pointed out in the appended claims.